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1 standard Acknowledgement: Slides borrowed from Richard Y. Yale

2 IEEE Requirements Design for small coverage (e.g. office, home) Low/no mobility High data rate applications Ability to integrate real time applications and non real time applications Use un licensed spectrum 2

3 802.11: Infrastructure Mode STA 1 ESS LAN BSS 1 Access Point BSS 2 Distribution System Access Point 802.x LAN Portal STA LAN STA 3 Architecture similar to cellular networks station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point access point (AP) station integrated into the wireless LAN and the distribution system basic service set (BSS) group of stations using the same AP portal bridge to other (wired) networks distribution system interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 3

4 IEEE Physical Layer Family of IEEE standards: unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz 300 MHz GHz GHz and b/g a 4

5 The IEEE Family Protocol Release Data Freq. Rate (typical) Rate (max) Legacy GHz 1 Mbps 2Mbps? Range (indoor) a GHz 25 Mbps 54 Mbps b GHz 6.5 Mbps 11 Mbps g GHz 25 Mbps 54 Mbps n /5 GHz 200 Mbps 540 Mbps ~30 m ~30 m ~30 m ~50 m 5

6 802.11a Modulation 3 different physical layer implementation Frequency hopping spread spectrum Direct sequence spread spectrum IR Adaptive modulation BPSK: 6, 9 Mbps QPSK: 12, 18 Mbps 16 QAM: 24, 36 Mbps 64 QAM: 48, 54 Mbps 6

7 r Traffic services MAC Layer m Asynchronous Data Service (mandatory) exchange of data packets based on best effort support of broadcast and multicast m Time Bounded Service (optional) exchange of bounded delay service 7

8 MAC Layer: Access Methods DFWMAC DCF CSMA/CA (mandatory) collision avoidance via randomized back off ACK packet for acknowledgements DFWMAC DCF w/ RTS/CTS (optional) additional virtual carrier sensing: to avoid hidden terminal problem DFWMAC PCF (optional) access point polls terminals according to a list 8

9 CSMA/CA CSMA: Listen before transmit Collision avoidance when transmitting a packet, choose a backoff interval in the range [0, CW] CW is contention window Count down the backoff interval when medium is idle count down is suspended if medium becomes busy Transmit when backoff interval reaches 0 9

10 Backoff IEEE contention window CW is adapted dynamically depending on collision occurrence after each collision, CW is doubled thus CW varies from CWmin to CWmax b a g aslottime 20 usec 9 usec 20 usec (mixed); 9 usec (g-only) acwmin 31 slots 15 slots 15 slots 10

11 Congestion Avoidance: Example busy B1 = 25 wait B1 = 5 data B2 = 20 busy data B2 = 15 wait B2 = 10 cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 Q: how is the performance of a mixed mode b/g network? 11

12 RTS/CTS + ACK r r r r Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) Receiver acknowledges via CTS (if ready to receive) m CTS reserves channel for sender, notifying possibly hidden stations Sender can now send data at once, acknowledgement via ACK Other stations store NAV distributed via RTS and CTS sender DIFS RTS data receiver SIFS CTS SIFS SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS new contention data t 12

13 Inter Frame Spacing r Defined different inter frame spacing r SIFS (Short Inter Frame Spacing); 10 us in b m highest priority, for ACK, CTS, polling response r PIFS (PCF IFS); 30 us in b m medium priority, for time-bounded service using PCF r DIFS (DCF, Distributed Coordination Function IFS); 50 us in b m lowest priority, for asynchronous data service DIFS DIFS medium busy PIFS SIFS contention next frame direct access if medium is free DIFS t 13

14 Inter Frame Spacing b a g asifstime 10 usec 16 usec 10 usec aslottime 20 usec 9 usec 20 usec (mixed); 9 usec (g only) adiftime (2xSlot+SIFS) 50 usec 34 usec 50 usec; 28 usec 14

15 802.11: PCF for Polling (Infrastructure Mode) point coordinator polled wireless stations NAV medium busy PIFS D NAV SIFS SIFS U contention free period D contention period t D: downstream poll, or data from point coordinator U: data from polled wireless station 15

16 802.11b Frame Format preamble 2 Sync SFD PLCP header MAC Data CRC Preamble (192 usec; or optional 96 short version) - Sync: alternating 0s and 1s (DSSS 128 bits) - SFD: Start Frame delimiter: PLCH (Physical Layer Convergence Procedure) Header - payload length - signaling field: the rate info. - CRC: 16 bit protection of header 16

17 MAC Data Format r r r r Types m control frames, management frames, data frames Sequence numbers m important against duplicated frames due to lost ACKs Addresses m receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous m sending time, checksum, frame control, data bytes Duration/ Address Address Address Sequence ID number Frame Control Address Data 4 CRC bits Protocol Type Subtype To From More Retry Power More version DS DS Frag Mgmt Data WEP Order 17

18 18

19 10% HT, 10% partial HT, 80% perfectly sense each other A Testbed Result USRPs a Each run randomly picks an AP and two clients

20 Throughput Comparison CDF of concurrent flow pairs Perfectly Sense Partial Hidden Hidden Terminals Terminals Throughput

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